1. Field of the Invention
The present invention relates to silicon nitride sintered bodies having high strength at high temperatures, and a method of producing the same.
2. Related Art Statement
Heretofore, as a silicon nitride sintered body containing an oxide of a Group IIIa element including rare earth elements, a method of producing a sintered body has been disclosed wherein 85 mole % or more of Si.sub.3 N.sub.4 is mixed with 15 mole % or less of at least one oxide of Group IIIa series elements, shaped, and sintered in a non-oxidizing atmosphere, as described in Japanese Patent publication No. 48-7,486. A and a silicon nitride sintered body has been disclosed which consists of at least 50 wt. % of Si.sub.3 N.sub.4, not over than 50 wt. % of Y.sub.2 O.sub.3 or at least one oxide of La series elements, and 0.01-20 wt. % of Al.sub.2 O.sub.3, as described in Japanese Patent Publication No. 49-21,091.
However, there are problems in that the mere addition of a rare earth element to silicon nitride can not produce a silicon nitride sintered body having high strength at high temperatures, and that the addition of Al.sub.2 O.sub.3 results in low softening point and hence remarkably decreased high temperature strength of the crystal grains boundary phase, though the addition of Al.sub.2 O.sub.3 improves densification of the silicon nitride sintered body.
In order to solve the problem of high temperature strength, the applicant disclosed a technique in Japanese Patent Application Laid-open No. 63-100,067 wherein a desired composition and a desired quantity ratio of rare earth element is added to Si.sub.3 N.sub.4 powder to specify the crystal phase of the sintered body so as to achieve high strength at high temperatures.
However, the silicon nitride sintered body disclosed in the Japanese Patent Application Laid-open No. 63-100,067 has a problem in that though it achieves high strength to certain extent at high temperatures the strength is inferior to room temperature strength thereof. This is considered due to a small amount of glass phase remaining in the composition even after the crystallization of the grain boundary phase. In order to decrease the remaining glass phase, a method may be considered of calculating the entire amount of oxygen contained in the raw materials of silicon nitride into SiO.sub.2 amount by conversion, and enlarging the quantity ratio of rare earth element oxide to SiO.sub.2 in the raw materials so as not to leave glass phase in the sintered body as far as possible. However, the method has a problem in that the densified sintered body is difficult to produce.